Plastic films generally generate and accumulate static electricity by contact or friction with various materials, or by being stripped from various materials during production or use thereof since they are electrically insulative. Such accumulated static electricity creates many problems. For example, it causes dust to be attacked to and to lie on the surface of the film, and may provide a strong electric shock to the user. In extreme cases, discharge occurs, causing combustion or ignition of inflammable substances which can lead to disaster. In the case of photographic films, such discharge traces result in the formation of static marks which is a serious defect for the film.
The invention is intended to provide substantially transparent plastic films which are made antistatic in order to remove problems caused by electrostatic charges.
The substantially transparent plastic film of the invention has many applications, such as wrapping, drawing, and drawing animations, as well as a support for recording materials, e.g., a support for silver halide photographic materials, a support for electrophotographic materials, and a support for magnetic films.
Methods which are known for preparing antistatic plastic films including (a) a method in which antistatic agents are incorporated into the plastic film, and (b) a method in which surface coating is applied onto the plastic film.
In accordance with the former method (a), the antistatic effect is insufficiently obtained unless the amount of the antistatic agent being incorporated is increased. Incorporation of the antistatic agent in such large amounts, however, results in the deterioration of mechanical properties or transparency of the plastic film.
Methods falling within the category of the latter method (b) include: (1) a method in which polymeric substances, surface active agents or the like are coated as an antistatic layer, (2) a method in which electrically conductive substances, e.g., carbon and metal powder, are coated as an antistatic layer, and (3) a method in which metals or metal oxides are vapor-deposited. Polymeric substances and surface active agents which can be used in the method (1) above are described in detail in Hideo Marumo, Taidenboshi Zai (Antistatic Agents), Saiwai Shobo, Tokyo. These polymeric substances and surface active agents, however, are inferior with respect to antistatic capability at low humidities becuase their electrical conductivities vary depending on humidity. However, they can be easily coated. The carbon and metal powder as used in the method (2) are not suitable for use in the prevention of charging of plastic films which must be transparent since the antistatic layer becomes opaque when these materials are incorporated. Addition of such metal powder or carbon in large amounts in order to increase the electrical conductivity results in the deterioration of mechanical strength of the antistatic layer, and particularly in the deterioration of abrasion properties. With regard to method (3), there are known so-called vapor deposited transparent electrically conductive films which are prepared by vapor depositing indium oxide with tin doped thereon or tin oxide with antimony doped thereon (see, for example, Hyomen (Surface), 18, 440 (1980)). This method, however, suffers from disadvantages in that the method of production is complicated, production costs are high, and the abrasion properties are inferior.